A foam layer has been formed such that a composition of a fluororesin and the like is melt-molded while being foamed. In the foaming of the fluororesin-containing composition, a gas such as nitrogen has been used as a foaming agent, and boron nitride as a foam nucleating agent. Since a dielectric constant of a foam layer varies with a porosity of a foam layer, it has been required that a foam layer with a low dielectric constant is provided by increasing a porosity.
Boron nitride used as a foam nucleating agent is free from moisture absorption and does not have an adverse effect on electrical characteristics. Thus, it is an advantageous component as a foam nucleating agent used in forming an insulation foam layer of a coaxial insulated cable. However, cells formed are large, and this agent is especially unsatisfactory for forming fine cells required for a coaxial insulated cable in which a wire diameter of a central conductor is small and an insulation layer is thin. Accordingly, a foaming composition capable of forming finer cells and a foam thereof have been in demand.